The Roman legionnaires: pillars of the Roman army

In
the early days of Rome there were no professional legionnaires. Every
man with enough property qualified and could be enrolled by the
consul for the next campaign.
He had to bring his own equipment which depended on the class he
belonged to. Also their age played a role.

The youngest of these troops are armed with a sword, light javelins, and a
buckler. The buckler is both strongly made, and of a size sufficient
for security. For it is of a circular form, and has three feet in the
diameter. They wear likewise upon their heads some simple sort of
covering; such as the skin of a wolf, or something of a similar kind;
which serves both for their defense, and to point out also to the
commanders those particular soldiers that are distinguished either by
their bravery or want of courage in the time of action.

Hastati were young and
little experienced legionnaires

Principes were experienced and strong soldiers

Triarii were very
experienced but older legionnaires
They were all armed in the same manner:

The next in age, who are called the hastati, are ordered to furnish
themselves with a complete suit of armor. This among the Romans
consists in the first place of a shield of a convex surface; the
breadth of which is two feet and a half; and the length four feet, or
four feet and a palm of those of the largest size. It is composed of
two planks, glued together, and covered first with linen, and
afterwards with calves' skin. The extreme edges of it, both above and
below, are guarded with plates of iron; as well to secure it against
the strokes of swords, as that it may be rested also upon the ground
without receiving any injury. To the surface is fitted likewise a shell
of iron; which serves to turn aside the more violent strokes of stones,
or spears, or any other ponderous weapon.
After the shield comes the sword, which is carried upon the right thigh, and is called the Spanish
sword. It is formed not only to push with at the point; but to make a
falling stroke with either edge, and with singular effect; for the
blade is remarkably strong and firm. To these arms are added two piles
or javelins; a helmet made of brass; and boots for the legs. The piles
are of two sorts; the one large, the other slender.

The trarii did not have javelins but a spear.
The segmented harness was first introduced in the
imperial period.

In 107 bc Gaius-Marius
reorganized the Roman army:
He changed the milicien's army into a professional army.
He allowed proletarians to join and make it their profession. They
signed a contract for 16 years. After this period they received a
pension in the form of a piece of land.

This would have important consequences because from now on the
legionnaires would develop loyalty towards their commanding officers.
This would make possible the operations of Sulla or Ceasar, because the
soldiers were loyal to them more the to the state.
Marius also dictated that every soldier had to carrie his own
equipment, rations for several days and supplies as well as two
poles for building the palisade walls of the camp that was built after
every displacement of the army. This gave the legionnaires the nickname
of „Marius’
mules”
Plutarchus (Life of Marius):

On
the expedition he carefully disciplined and trained his army whilst they were on their way,
giving them practice in long marches, and running of every sort,
and compelling every man to carry his own baggage and prepare his own
victuals; insomuch that thenceforward laborious legionnaires, who did
their work silently without grumbling, had the name of "Marius' mules."